Communication devices such as wireless device are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
D2D communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. In D2D communications, wireless devices, such as user equipments, that are in the proximity of each other discover one another, in a process referred to as device discovery. They establish a direct link referred to as D2D bearer establishment, rather than a link via a base station. The initiation of the establishment of the D2D link may be made by the radio access network or by any of the wireless devices of the D2D pair. Examples of D2D communications include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These systems operate in unlicensed spectrum.
Wireless devices that want to communicate, or even just discover each other, typically need to transmit various forms of control signaling. One example of such control signaling is the so-called signal or discovery signal, which may possibly include a full message. The signal carries at least carries some form of device identity and is transmitted by a wireless device that wants to be discoverable by other wireless devices. Other wireless devices may scan for the discovery signals. Once they have detected the discovery signal, they may take the appropriate action, for example to try to initiate a connection setup with the device transmitting the discovery message.
Multiple discovery signals from different wireless devices are multiplexed on the same radio resources in a combination of Time Division Multiplexing (TDM), Frequency-Division Multiplexing (FDM) and possibly Code Division Multiplexing (CDM). Even though details are not agreed yet, it is likely that discovery signals may be multiplexed on specific subframes occurring at known, or signaled, positions in the radio frame. Such subframes carrying at least discovery signals are called discovery subframes in the following.
So far, 3GPP RAN1#73 R1-132861 has agreed to study further the following options for scheduling discovery signals:                Type 1: A discovery procedure where resources for discovery signal transmission are allocated on a non-wireless device specific basis, e.g., non-UE specific basis,                    Note: Resources may be for all wireless devices or group of wireless devices.                        Type 2: A discovery procedure where resources for discovery signal transmission are allocated on a per wireless device specific basis.                    Type 2A: Resources are allocated for each specific transmission instance of discovery signals.            Type 2B: Resources are semi-persistently allocated for discovery signal transmission.                        
While type-1 and type-2 were defined, so far there is no agreement about when to use which type.
Recently, D2D communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating wireless devices and, at the same time, to allow wireless devices to operate in a controlled interference environment. Typically, it is suggested that such D2D communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated spectrum for D2D purposes is a less likely alternative, as spectrum is a scarce resource and, dynamic, sharing between the D2D services and cellular services is more flexible and provides higher spectrum efficiency.